This invention relates to a conductor paste, a process for producing it, circuit substrates using such a conductor paste, and electronic circuit modules comprising semiconductor elements mounted on such substrates in operative arrangement.
In the art of electric circuits, remarkable progress has been made in packaging density and integration of electronic parts in recent years. In line with such progress in the art, the request has become stronger for higher density circuit substrates on which the electric parts are mounted, and many intensive studies have been made for higher wiring density and multi-layer lamination in the manufacture of substrates. Particularly in memory and logic devices having a multiplicity of highly integrated LSI chips mounted compactly therein, such as electronic computers, progress in techniques for wiring density and multi-layer structure of substrates is remarkable. Hitherto, the highest density packaging has been achieved with the multilayer wiring substrates of the green sheet system. In the manufacture of this type of substrate, a conductor paste, especially a paste containing a high-melting point metal powder, such as tungsten paste or molybdenum paste, is generally used for forming conductor wiring patterns. In these conductor pastes such as tungsten paste, as for instance mentioned in Japanese Patent Application Laid-Open (Kokai) No. 62-143981, the content of metal powder in the paste is mostly in the range of 75 to 85% by weight, and the specific resistance (resistivity) of the conductor after printing and baking by using the paste is about 20 .mu..OMEGA..cm in both cases of tungsten and molybdenum pastes.
In case of producing a conductor having a resistivity of 0.5 .OMEGA. or less per cm by using this type of conductor paste, it is usually tried to form a conductor having the dimensions of about 150 .mu.m in width, about 40 .mu.m in thickness and about 4,000 .mu.m.sup.2 in effective sectional area (the sectional shape being close to rhomb).
For obtaining a conductor thickness of 40 .mu.m, it is necessary to apply the paste so that the paste thickness after circuit pattern printing and drying will become 50 to 60 .mu.m by taking into consideration possible slackening at the time of sheet lamination and shrinkage at the time of sintering. This makes the printing operation relatively difficult to perform. It is also practiced to form a paste using a powder of a metal such as tungsten which has been pulverized to an average particle size of 1 .mu.m or less for the purpose of lowering resistivity. This scheme has succeeded in reducing resistivity of the conductor after printing and baking to about 15 .mu..OMEGA..cm. This low resistivity paste, however, is substantially incapable of thick printing and the film thickness can reach only the order of 20 to 30 .mu.m at the greatest after baking. After all, the resistivity per unit length becomes equal to or even less than that of the conventional pastes. One reason for this phenomenon will be that the surface area of the metal component in the paste increases due to pulverization of metal powder, and this causes a corresponding increase of the amount of vehicle, resulting in a reduced metal content in the paste.
Under these circumstances, the request for higher wiring density is ardent, and now circuit patterns with a conductor width of less than 100 .mu.m have become necessary in the art. In case the conductor width is reduced from 150 .mu.m to 100 .mu.m, if the film thickness after baking is made 60 .mu.m, it is possible to realize the same paste as the conventional ones, only in calculations. Actually, however, if the conductor width is made 100 .mu.m, the upper threshold value of the film thickness allowable for a conventional paste is about 30 .mu.m, which means that it is impossible to obtain a film having greater than about half of the desired thickness. This is mainly due to slackening of conductor paste at the time of printing, collapse or lateral spreading of the wiring conductors when laminating the sheets, and shrinkage at the time of sintering. Therefore, in order to acquire the same resistivity as the conventional pastes even in high density packaging, there is required a paste whose specific resistance after baking is about 10 .mu..OMEGA..cm, which is half of that of the conventional pastes, and also capable of providing a film thickness of 25 to 35 .mu.m after baking. It has been impossible to meet the-se requirements with the prior art technology.